Ions Measured by Voyager 1 Outside the Heliopause to 28 AU

Following the crossing of the Heliopause (HP) by Voyager 1 (V1), the expectation was that in addition to the cold interstellar (IS) plasma, there may well be a superthermal particle population that could play a role in the pressure balance between the heliosphere and the upstream medium (Gloeckler et al. 1997-doi:10.1038/386374a0). The precipitous decrease in solar material to apparent background levels observed at the time of exit (Krimigis et al. 2013-doi:10.1126/science.1235721), indicated that no such population existed. We now show that such a suprathermal ion population exists at a level of ~1% from that in the heliosheath. Specifically, we report 40-139 keV in-situ ion measurements from V1/Low Energy Charged Particle (LECP) instrument, over the time period from 2000 to the first three months of 2020, towards assessing energetic ion flow properties inside the heliosheath and upstream of the HP. We identify an average radial outflow of ions at ~46, 67 and 109 keV of ~2.33×10-3, ~1.37×10-3 and ~7.27×10-4 /(cm2·sr·s·keV), respectively, from the beginning of 2013 up to 2020, i.e. over a spatial range of ~28 AU past the HP. The intensity spectrum is consistent with a power law form in energy with a spectral index of ~-1.4, similar to energy spectra measured by both V1 & V2/LECP inside the heliosheath, providing strong indication that these particles correspond to an ion population leaking from the heliosheath into interstellar space, most likely due to the flux tube interchange instability at the boundary (Krimigis et al. 2013-doi:10.1126/science.1235721; Florisnki, 2015-doi:10.1088/0004-637X/813/1/49). Future modeling efforts should incorporate this ion population, that provides the framework for establishing a communication between the heliosheath and the Very Local Interstellar Medium.